Microphone circuit



y 1945- H. c. HORNICKEL 8 ,438

' MICROPHONE CIRCUIT Filed Feb. 11, 1944 INVENTOR. FIG 3 HERMAN c. HORNICKEL ATTORNEY Patented July 31, 1945 UNITED STATES PATENT, A orr ca A mcsm ifz im I I i Herman Conrad Hornle kel, Fair Haven. N. J. I Application February 11. figs-1) o.

(Granted under the act of March 3. 1883.v as amended April 80, 1028; 870 0. G. 757) 2 Claims.

The invention described herein may be manulectured and used by or for the Government for govemmental purposes, without the payment to me of any royalty thereon. v

This invention relates to electro-acoustic transducer circuits and more especially to'input circults for carbon microphones.

Due to its high sensitivity the carbon microphone is almost universally employed in radiotelephone communication circuits, wherein the primary requisite is sensitivity rather than uniform frequency response over a broad frequency range. Voice frequencies falling within a range of about 100-4009 cycles are adequately reproduced by carbon microphones, but in fidelity systems, such as radio broadcasting or motion picture recording, an increased frequency response is desirable and other types 01' microphones. as for example, the condenser or velocity microphones, are preferably used. g

Carbon microphones fall into two classes; that of the single-button type and that oi the doublebutton type. In the single-button microphone the voice waves impinge upon a metal diaphragm having mounted in the central area thereon a button comprising a hemispherical metallic cup filled with carbon granules, usually manufactured of anthracite coal, and some form of closure to prevent carbon leakage at the point where they make contact with the diphragm. As the diaphragm is displaced, the pressure is varied between the granules in the cup which results in a change of resistance i'rom granule to granule. which, in turn, results in a corresponding change in resistance between the cup and diaphragm. By connecting the microphone in series with a source of electrical energy these resistance changes are converted into corresponding electrlcal fluctuations. In the two-button microphone an additional cup is mounted on the diaphragm on the opposite lace thereof, and the microphone operates similarly, but in an electrical push-pull manner.-

As regards fidelity. the double-button carbon microphone is known to have numerous advantages over the single-button type. Improved frequency response is obtained in the double-button microphone due to balanced diaphragm loading. Because of the push-pull operation or the doublebutton microphone the even order harmonics generated in the buttons arecancelled, thus eii'ecting a further reduction in distortion. The variations in frequency response and harmonic content as a resultoiaehangeinthediaphragmposition which occur in a single-button microphone are minimized in the double-button type.

Another desirable feature oi the double-button microphone or primary importance in military applications is that it may be successfully adapted to the provision of noise cancellation. Noise cancellation is accomplished by introducing ambient noise to both sides of a microphone diaphragm whereas the voice impinges only on one side. It has been found that the use oi a double-button microphone will allow improvednoise cancellation due to the fact that bothsides oi the diaphragm. will be loaded equally and the acoustic chambers on both sides will be identical.

Because oi the simplicity and relative inex pensiveness of thesingle-button microphone, as well as its excellent sensitivity, a large majority or radiophone and intercommunication 8913813.

tus employed for military and other purposes entailing voice transmission have been designed for single-button carbon microphones. These microphones may be carried in the i'orm of handsets, chestsets, iace mountings and throat mountings.

and are ordinarily linked to-the communication apparatus by a two-wire cord. Inasmuch as the double-button microphone requires a three-wire line and a center-tapped primary microphone transformer, the double-button microphone is not interchangeable with the single-button type without alterations in the wiring and the components 01' the input circuit.

Communications equipment in recent years has been improved to a degree where the limiting. factor in the transmission of intelligence is the single-button carbon microphone. Although the communications apparatus may be capable oi uniform frequency response 01' a wide audio range,

the full effectiveness oi the apparatus is not employed' since the over-all iidelity o! the apparatus never exceeds the characteristics or the weakest element in the system, which in this instance, is the single-button'microphone. As the percentage of articulation is contingent upon the amount 0! noise and distortion present in a communications system, it is manifest that the service area covered by a radiotelephone transmitter may be augmented by improved microphone response. However. inasmuch as vast quantities oi communications apparatus'which are presently in iield use 7 employ two-wire microphone input circuits intended iorsingle-button carbon microphones. the revision of circuit design in order to awesome-'- date a double-button microphone having a threewire line would involve considerable time and ezpenseiiperi'ormedonalargeeeale.

vention is to provide an external circuit and apparatus, whereby a double-button carbon microphone may be adapted as a direct replacement for a standard single-button microphone without any alterations in microphone cordage, microphone plugs, or of the input circuits with which the single-button microphone is associated.

Another object of my invention is to provide a circuit of the'above type whereby the full fidelity response of a double-button microphone is obtained in a single-button channel.

A further object of my invention is to provide a circuit of the above type in which all the requisite apparatus may be contained in the microphone housing.

The various objects and features of my invention will be better understood from the following detailed description when read in conjunction with the accompanying drawing in which:

Figure '1 illustrates the schematic circuit diagram of a conventional form of single-button .carbon microphone input circuit;

identical parts. Referring now to the drawing,-

and more particularlyto Figure l. a single-button carbon microphone ID is shown connected to the input terminals ll of a conventional microphone amplifier l2, a portion of which is shown. The connection to the amplifier I2 is made by means of a two-wire cord l8 and the microphone may be contained in any form, that is, in a handset, throat mounting. etc., with a switch it mounted on the handset or mounting structure. An input transformer I is provided, having a primary winding whose impedance matchesthat of microphone lfl. A battery It is'connected in series with microphone l0 and primary winding of transformer ll to furnish energy for the microphone circuit. The .voltage from battery is controlled by a potentiometer II. The circuit illustrated in Figure 1 is a commonly used form of microphone input circuit and my invention is applicable to this form or any other conventional form of single-button microphone input circuit. whether the input be resistive or inductive.

Referring now to Figure 2, a circuit is shown for adapting the two-wire line for use with a double-button microphone. The components of the circuit are a center-tapped primary, microphone transformer it and a double-button carbon microphone ll, both components being preferably contained in a single housing or mounting. The output of this circuit is fed to the identical twowire cord I! and amplifier i2 illustrated in Figure between the diaphragm and the center-tap of the primary winding. This would ordinarily require the use of a third wire from the amplifier II, but I have obviated the need for this wire by my circuit arrangement wherein it may be seen that one terminal of the battery ll is connected through potentiometer H to the diaphragm 0f microphone ll, whereas the other terminal is connected to the center-tap of primary winding of transformer is through the primary winding of transformer ll.

and secondary winding of transformer It. The series resistance offered by the primary of transformer II and the secondary of transformer ll requires a higher voltage to secure the proper mlcrophone current. This voltage can be readily oretaified by the proper positioning of potentiome r The audio output of microphone it appears across the secondary windin of transformer I8, and one end of the secondary winding is wired directly to one ofthe amplifier input terminals II. The opposite end of the secondary connected to the center-tap of primary winding of transformer I, where it is connected toone of the terminals ll through the microphone ll. Since the primary microphone circuit of transformer I8 is of the balanced type, it does not introduce a variable resistance into the secondary output circuit, so that, in effect, the secondary of transformer ll is connected to the input terminals ll through a small fixed series resistance.

For the purpose of eliminating the direct current of battery I 0 from the secondary of transformer I 9, I have shown another embodiment of my invention in Figure 3, wherein a choke 20 is provided having a high impedance to audio frequency currents. Energy for the microphone is is obtained from the battery l8 through the primary of transformer is in series with choke Ill.

Inasmuch as the condenser 2i serves as an open i. The end leads of the primary winding of transformer II are connected to the buttons of microphone l8 and the center-tap of the primary winding is connected to one end of a secondary or transformer It. The other end of a secondary winding of transformer II is connected to one of the input terminals ll of amplifier If. The remaining amplifier terminal II is connected through the cord II to the diaphragm of microphone II.

In conventional double-button microphone circuit to direct current, the primary current does not flow through the secondary of transformer ll, thereby eliminating distortion due to saturation effects in the core. The output of transformer ll, being of an alternating current nature, is conducted by condenser 2| to one of the terminals Ii and is not affected by choke 20 since it functions as a high impedance to audio frequency currents.

The components shown in'Figures 2 and -3 may be assembled in a unitary structure in the form of a handset or chest mounting or whatever structure is desired. The transformer it and choke II are preferably of a miniature lightweight construction.

It is to be noted that no changes have been made in the amplifier It or in the microphone extension cord il so that it is merely necessary to connect the two-wire cord of any single-button microphone channel to the present invention in order to convert it for double-button microphone use and obtain its attendant advantages.

Various modifications of the circuits. which have been illustrated and described, which are within the spirit and scope of the invention. will occur to persons skilled in the art.

What is claimed is:

1. The combination in a circuit for adapting a double-button microphone to a two-wire input circuit arranged for a single-button microphone and including a source of electrical energy. comprising, a transformer having a center-tapped primary winding, a secondary winding. one end or said secondary winding being connected to the transformer circuits the supply voltage is applied 1 center-tap of said primary winding and the other end or said secondary winding being connected to one end or said input circuit, a double-button carbon microphone having its diaphragm connected to the end of said input circuit and its buttons connected to the primary winding of said transformer whereby said double-button microphone is electrically energized and in electrical association with said single-button input circuit.

2. The combination, in a circuit for adapting a double-button microphone to a two-wire input circuit arranged for a single-button microphone and including a source of electrical energy, comprising, a transformer having a center-tapped primary winding, a secondary winding, one end 01 said secondary winding being connected to the across the secondary of said transformer and having its juncture in connection with the other end of said single-button input circuit, whereby said double-button microphone is electrically energized and in electrical association with said single-button input circuit.

HERMAN CONRAD HORNICKEL. 

